Those fatty acid esters derived from vegetable fat and oil are used as a cooking oil and, in addition, they are used in such fields as cosmetics and pharmaceuticals. In recent years, attention has been paid to them as additives to fuels such as light oil. For example, they are added as vegetable-derived biodiesel fuel to light oil at addition levels of several percent for the purpose of reducing the emission of CO2. Glycerine is mainly used in such various fields as a raw material for the production of nitroglycerin and is further used as a raw material for alkyd resins, pharmaceuticals, foods and printing inks and cosmetics. It is known that a method of producing such fatty acid esters and/or glycerine by transesterification of triglyceride, which is main component of fats and oils, with a lower alkyl alcohol.
In carrying out such a production method on a commercial scale, a homogenous alkali catalyst is generally used. This, however, makes it necessary to carry out a complicated step of separation and/or removing the catalyst. Further, the alkali catalyst causes saponification of a free fatty acid contained in a fat/oil to form soaps as byproducts, whereby it becomes necessary to carry out a step of washing with large amounts of water and, in addition, the yield of alkyl esters decreases due to the emulsifying effect of the soaps and, in certain instances, the subsequent glycerine purification process becomes complicated.
Concerning the conventional method of catalytic transesterification of fatty acid glycerides (a fat/oil), Japanese Kokai Publication S61-254255 discloses a method comprising reacting fatty acid glycerides with a lower alcohol and then separating the free glycerine wherein sodium carbonate and/or sodium hydrogencarbonate is used as a heterogeneous solid catalyst. It is described that, in carrying out this method, the alcohol is evaporated from the reaction mixture and then the layer containing the free glycerine is separated and removed. It is also described that when the reaction is carried out continuously, the free alcohol included in the reaction mixture is partly evaporated and then the heavy glycerine phase is separated and removed by phase separation, part of the light ester phase is returned to the transesterification step as a recycling stream and the evaporated alcohol and fresh portions of the reactants are simultaneously introduced into the same step. However, the transesterification method using such a catalyst has problems, namely the alkali catalyst may be converted into a soap when a free fatty acid coexists in the raw material fat/oil, or the catalyst may be eluted by the water contained in the fat/oil or, when the active species of the catalyst is eluted, the reverse reaction may proceed in the step of evaporation of the alcohol, resulting in decreases in yield. Therefore, there is room for contrivance for enabling the production of high-purity fatty acid lower alkyl esters and glycerine in high efficiency, by prolonging the catalyst life-time and/or suppressing the reverse reaction resulted from the elution of the active species of a solid catalyst to thereby carry out the transesterification reaction with high efficiency.
Further, regarding a continuous producing method of fatty acid esters and glycerine, Japanese Kokai Publication 2001-31991 discloses that a fat/oil and an alcohol are preheated and reacted in the absence of catalysts where the preheating temperature and reaction temperature are not lower than the critical temperature of the alcohol and the preheating pressure is not lower than 0.7 MPa, and then the alcohol is evaporated from the reaction mixture obtained, follow by light liquid phase containing fatty acid esters and heavy liquid phase containing glycerine are separated from each other. There is no description of the use of a catalyst in carrying out this method; the reaction is carried out at high temperatures and under high pressure.
Furthermore, Japanese Kokai Publication 2003-104935 discloses a method of producing fatty acid esters which comprises reacting a fat/oil with a monohydric alcohol under supercritical condition of the alcohol, and feeding the reaction mixture containing the unreacted materials and/or intermediate products to a reactor. In this process as well, the reaction is carried out at high temperatures and under high pressure. It is described, in the examples, that a methanol slurry containing a MnO2 powder suspended therein is fed and that the monohydric alcohol is fed in an amount of about 17 times the theoretical amount to be fed. However, there is still room for contrivance for rendering these methods capable of producing high-purity fatty acid lower alkyl esters and/or glycerine at low cost by improving the conversion and reducing the energy consumption of the production process in an advantageous manner from the energy viewpoint.
Further, Japanese Kokai Publication 2002-294277 discloses a method of producing a lower alkyl ester by transesterification of a fat/oil with a lower alcohol in the presence of a catalyst, which comprises using a catalyst containing a composite metal oxide having a perovskite-type structure. This composite metal oxide containing a perovskite-type structure is highly basic and preferably contains a cesium (Cs) compound. The cesium (Cs)-containing one includes Ca—, Sr— and/or Ba-containing ones. However, when such composite metal oxides having a perovskite-type structure are used, high reaction temperature is required, since pure perovskite is low in activity. High reaction temperatures allow the elution of the active ingredients of the catalysts. When calcium oxide and/or a cesium component occurs externally to the perovskite crystal lattices, the activity increases and the catalyst allows the reaction to proceed even at ordinary pressure. However, a problem arises; such active metal components as Ca and Cs are eluted into the liquid reaction mixture in large quantities.